Polymer dispersed ferroelectric smectic liquid crystal formed by inducing a force during phase separation

ABSTRACT

Disclosed is a class of light modulating materials comprising microdomains of ferroelectric smectic liquid crystal dispersed in a light-transmissive polymer medium. The microdomains are formed by phase separation of the liquid crystal from solution with the polymer as the polymer is solidified. The switching of the liquid crystal may be either monostable or multistable (e.g., bistable), depending on the liquid crystal and polymer. The material modulates light in either a scattering-transmissive mode or a birefringence mode. Materials operating in the scattering-transmissive mode do not require polarizers, Advantages of the materials include switching times down to the order of microseconds or less, multistable optical states, wide viewing angles and high contrast.

This is a division of application Ser. No. 07/950,785, now U.S. Pat. No.5,321,533, filed Sep. 24, 1992.

TECHNICAL FIELD

The present invention relates generally to liquid crystal technology,and more specifically to the manufacture of novel ferroelectric smecticliquid crystal dispersed in a polymer matrix providing an electro-opticeffect offering multistable optical states, fast switching times down toa few microseconds or less, high contrast and brightness, and wideviewing angles.

BACKGROUND OF THE INVENTION

Smectic liquid crystals are characterized by molecules which align inlayers. Molecular ordering exists within each layer, with the degree ofmolecular ordering depending on the particular smectic phase. In chiralsmectic materials, which include the chiral smectic C*, F*, I*, G* andH* phases, the ordering of the molecules within the layers rotates by aconstant angle from layer to layer, so that the liquid crystal structureis twisted. For example, smectic C* phases are characterized in that themolecules align in layers in which the directors (i.e., the commondirections of the long axes of the molecules within each layer) areoblique to the layer boundaries, so that the "tilt angle" of themolecules (i.e., the angle between the director and the layer normal) isthe same from layer to layer. In bulk samples of smectic C* material,the directors twist from layer to layer.

Molecules of substances having chiral smectic liquid crystal phases havepermanent dipole moments approximately normal to the director. Thealignment of molecules of chiral smectic liquid crystal in an externalelectric field is determined in part by competition between the torquesinduced by this permanent dipole moment and by the induced dipole momentwhich, in materials with positive dielectric anisotropy, is parallel tothe director. The smectic C*, F*, I*, G* and H* phases are all"ferroelectric" in the sense that geometry of the liquid crystal may bealtered with the application of an electromagnetic field, and then,under proper conditions, remains stable once the field is removed.

A number of transmissive mode displays using smectic C* phase liquidcrystal have been proposed. For example, "surface-stabilizedferroelectric liquid crystal" ["SSFLC"] displays make use of thin filmsof smectic C* liquid crystal confined between parallel substrates, InSSFLC displays, the liquid crystal is preferably aligned in a so-called"bookshelf" geometry in which the molecules are arranged in layers thatare perpendicular to the inner surfaces of the substrates and themolecules are approximately parallel along a 45° angle to the layers.The material may be switched between two stable orientations bygenerating external electric fields across the liquid crystal normal tothe inner surfaces of the substrates. The field useful for switching thematerial from one orientation to another is opposite in polarity to thefield useful for switching back to the first of the orientations. If thefilm is sufficiently thin, each orientation should be stable when theelectric field is removed. The two stable orientations differ in thatthe directors in the two orientations form mirror images about a planenormal to the layers and to the inner surfaces of the substrates.

In one proposed SSFLC device, the liquid crystal is confined betweenparallel substrates which are placed between a polarizer and ananalyzer. The device modulates light by controlling the polarizationdirection of linearly polarized light transmitted through the liquidcrystal. Due to the birefringence of the liquid crystal light incidenton the liquid crystal is decomposed into two orthogonally-polarizedcomponents having different speeds. The relative advance of one of thepolarized components relative to the other rotates the polarizationdirection of the transmitted light relative to that of the incidentlight to a degree dependent on the thickness of the liquid crystal filmand the orientation of the director within the film.

In a preferred device, the substrates contain a film of liquid crystalhaving a film thickness selected to effect a 90° rotation of thepolarization direction of the light incident on the liquid crystal. Ifthe polarizer is aligned at approximately 45° with respect to the opticaxis of the liquid crystal in the first of the two orientations and theanalyzer is oriented parallel or perpendicular to the polarizer, thelights transmission through the device is maximized or minimized in thefirst stable orientation of the liquid crystal. Applying an electricfield to transform the liquid crystal to the second stable orientationchanges the intensity of light transmitted by the device. Consequently,such an SSFLC display is bright when the liquid crystal is in one of thestable orientations and dark When the liquid crystal is in the otherorientation. The optical behavior of the device may be changed byrotating the analyzer relative to the polarizer, or either the polarizeror analyzer relative to the liquid crystal.

Among the advantages attributed to displays using smectic C* liquidcrystal are fast switching times, high contrast and wide viewing anglescompared to commercially available liquid crystal displays such astwisted nematic displays. One application for these ferroelectric liquidcrystal materials would be in computer display terminals andtelevisions. Most currently available flat panel displays are based ontwisted nematic liquid crystal. These flat panel displays require lesspower than conventional cathode ray tubes, but have not replaced cathoderay tubes due to their slow response, poor contrast, low brightness andnarrow viewing angle.

Displays using ferroelectric smectic C* liquid crystal may be capable ofswitching times on the order of microseconds or less, whereas theswitching times of twisted nematic displays are often on the order ofmilliseconds. Existing SSFLC displays have shown viewing angles inexcess of 45° and contrast ratios on the order of 1500:1, which exceedthe performance of typical twist cells. Despite these advantages, SSFLCdisplays have not replaced cathode ray tubes due to the technicaldifficulty and expense of obtaining stable bookshelf alignment of theliquid crystal. Furthermore, the use of surface coatings with stronganchoring which promote alignment of the liquid crystal parallel to thesubstrates increases the switching voltage required to switch betweenstable orientations. Another disadvantage inherent in SSFLC displays isthat the substrate anchoring that is necessary for bookshelf alignmentis unstable; the liquid crystal may switch to light scattering "zig-zag"or "chevron" texture if jarred rendering the display worthless. Despiteintense research over the past decade, there remains a need for aneconomical method for aligning ferroelectric smectic liquid crystal in ageometry useful for display applications.

Flat panel liquid crystal displays using a nematic liquid crystal phasehave been formed by phase separation of the liquid crystal phase fromsolution with a polymer or pre-polymeric resin. The earliest form ofthese materials comprised microdroplets of liquid crystal dispersed in acontinuous polymeric matrix. In such materials, the ordinary index ofrefraction was matched to an index of refraction Of the polymer. Thematerial scattered light in the absence of an external field andtransmitted light in the presence of an electric field. The evolution ofsuch materials may be found in references such as U.S. Pat. Nos.4,671.,618; 4,673,255; 4,685,771; 4,688,900; 4,890,902; 5,004,323 and5;093;735, the disclosures of which are incorporated by reference.

Three techniques have been proposed for inducing phase separation of thenematic liquid crystal phase from the polymer phase. According to amethod known as polymerization induced phase separation or "PIPS," theliquid crystal is dissolved in a prepolymer followed by polymerization.According to another method known as "thermal induced phase separation"or "TIPS," the liquid crystal is dissolved (or redissolved) in a polymermelt followed by cooling. According to the third method, known as"solvent induced phase separation" or "SIPS" the liquid crystal andpolymer are dissolved in a common solvent followed by evolution of thesolvent. The polymer is often cross-linked to improve the properties ofthe display material. The size and density of the droplets may be variedby changing the ratios of the liquid crystal and polymer phases as wellas by charging the conditions under which phase separation occurs,

While flat panel displays comprising material formed by phase separationof a nematic liquid crystal phase from solution with a polymer appear tobe highly durable, as Well as useful and economical for manyapplications, the fastest switching times for such materials remain onthe order of a millisecond. Furthermore, the viewing angles for thesematerials can be increased beyond about 20° only through the use ofspecialized birefringent polymers which increase the cost of thedisplays. There remains a need for relatively inexpensive flat paneldisplays with higher switching speeds, higher contrast and largerviewing angles.

DISCLOSURE OF THE INVENTION

This invention provides a new class of liquid crystal light modulatingmaterials comprising a ferroelectric liquid crystal phase interspersedwith a light transmissive polymer phase. The new materials of theinvention are characterized in part by simplicity of preparation anddisplay fabrication, switching times on the order of microseconds,multistable switching, high contrast and wide viewing angles.

A preferred form of the material includes microdomains of ferroelectricsmectic liquid crystal dispersed in a continuous matrix oflight-transmissive polymer. Within the microdomains, the liquid crystalis aligned such that the optical behavior of the material changes whenthe material is exposed to an external electric field (AC, DC or acombination thereof) normal to a viewing surface. If the microdomainsare sufficiently small, the alignment will be stable even when the lightmodulating material is jarred. Furthermore, bistable or multistableswitching of the liquid crystal is possible if the anchoring of theliquid crystal at the boundaries of the microdomains is sufficientlyweak.

According to a preferred embodiment, the material is fabricated by phaseseparation of a ferroelectric smectic phase from solution with a polymeror prepolymer in the presence of a force promoting an alignment of theliquid crystal, such as shear stresses and forces generated by anexternal electromagnetic field or a temperature gradient. While phaseseparation of the liquid crystal may be induced either by the PIPS orTIPS methods, good results have been obtained by dissolving a liquidcrystal having a ferroelectric smectic phase with a polymer in a solventand then inducing phase separation by evaporation of the solvent.

Either thermosetting or thermoplastic polymers may be used, includingpolystyrenes, polymers of α-methylstyrene, vinyl-toluene, isobutylene,butadiene, methylbutene, or epoxies cured with various curing agentssuch as thiols, alcohols and mercaptans. Birefringent polymers may beused, depending on the application.

Likewise, preferred liquid crystals include those having ferroelectricsmectic phases near room temperature or another working temperaturerange for the material, and which are compatible with the polymer. Amongthese are ZLI-3654, ZLI-4003, ZLI-4005, ZLI-4140, and ZLI-4237-100available from E. Merck of Darmstadt, Germany; SEC-13 and 842, availablefrom BDH; a mixture of W7 and W3, each available from Displaytech; AlphaChloro Ester (S5, R-6) available from Aldrich Chemical; and a materialwith the structural formula C₂ H₅ --CCH₃ H--CCIH--COO--Ph--Ph--OR andits mixtures. Liquid crystal having positive or negative dielectricanisotropy may be used, depending on the application.

Particularly useful materials feature spherical or spheroidalmicrodroplets of liquid crystal having diameters of the order ofmagnitude of the pitch length of the liquid crystal or less. Suchdroplets form spontaneously if phase separation is performed underappropriate conditions. For example, a cross-linking agent may be addedto the solution of liquid crystal and polymer, and cross-linking of thepolymer phase initiated at an appropriate time to limit the growth ofthe liquid crystal droplets. Droplet size may also be controlled byheating a phase separated light modulating material in order toredissolve the liquid crystal and polymer, and then cooling the solutionat a controlled rate in order to reinduce phase separation. These andother techniques for controlling the size and shape of liquid crystalmicrodroplets formed during phase separation are discussed in U.S. Pat.No. 4,673,255, the disclosure of which is incorporated by reference.

When phase separation of a ferroelectric smectic material occurs in thepresence of a force promoting the alignment of the liquid crystal in themicrodomains, such as an external electromagnetic field, applied shear,temperature gradient or a combination thereof, the liquid crystalassumes a geometry featuring parallel layers analogous to bookshelfgeometry in thin films of the liquid crystal. (For convenience, theparallel layer geometry without twist in microdomains will also bereferred to as "bookshelf" geometry.") If the anchoring between theliquid crystal and polymer is sufficiently weak, the microdomains may beswitched bistably or multistably, depending on the smectic phase.Several switching modes, including normally scattering, normallytransmitting and bistable modes, are possible depending on thecomposition of the material and the direction of the alignment promotedby the force applied to the material during, or after, phase separation.

Preferably, the materials modulate light by means of one of twomechanisms. In "scattering-transmissive" devices, the effective index ofrefraction of the liquid crystal is matched to an index of refraction ofthe polymer in one alignment of the liquid crystal, and mismatched withthat refractive index of the polymer in another alignment of the liquidcrystal. When the alignment of the liquid crystal is such that theeffective indices of refraction of the liquid crystal and polymer arematched, incident light is transmitted through the material on the otherhand, when the alignment of the liquid crystal is such that theeffective indices of refraction of the liquid crystal along the cellaxis (perpendicular to the substrates) and polymer differ significantly,incident light is scattered.

In the "birefringent" mode, the index of refraction of the polymer andthe index of refraction in the liquid crystal along the cell axis match.The liquid crystal film thickness and the orientation of the liquidcrystal in the microdroplets are selected to rotate the polarizationdirection of the incident beam by a substantial angle as the lighttraverses through the cell. The orientation of the polarizer on the oneside of the cell and the analyzer on the other side are adjusted to givethe minimum transmission in one of the two orientations of the liquidcrystal. The other orientation, then, corresponds to maximumtransmission.

In both modes, the boundary conditions and the electrical properties ofthe liquid crystal can be selected to form a monostable, bistable ormultistable device. Applications of such devices includes highresolution display devices, optical computing, optical data storage andoptical communications.

One object of the invention is the formation of improved lightmodulating materials comprising microdomains of ferroelectric liquidcrystals dispersed in a polymer medium still other features andadvantages and a full understanding of the invention will becomeapparent to those skilled in the art from the following description ofthe best modes and the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic cross-sectional views showing oneembodiment of the material in an as-formed (or, in the case ofmonostable materials, a field-OFF condition) and a field-ON condition,respectively;

FIG. 2A is a schematic cross-sectional views of a second embodiment ofthe material in an as-formed (or, in the case of a monostable material,field-OFF) condition; and

FIGS. 2B and 2C are schematic cross-sectional views of the embodiment ofFIG. 2A in different field-ON conditions. While polarizers are shown inFIG. 2C for purposes of consistency with FIGS. 2A and 2B, they areunnecessary to the operation of a display making use of the conditionshown in FIG. 2C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1A-2C illustrate various embodiments of a light modulatingmaterial comprising microdomains of ferroelectric smectic liquid crystalformed in a medium of light transmissive polymer by phase separationfrom solution with the polymer or a prepolymer. For example, FIGS. 1Aand 1B illustrate a shutter or display 10 comprising a light modulatingmaterial in which droplets 12 of the liquid crystal form in a polymermedium 14 during phase separation in a magnetic field normal to aviewing surface 16 of the material, The device 10 includes, in additionto the light modulating material, transparent substrates 18, 20(preferably of glass) having transparent electrodes 22, 24 (preferablyof indium-tin-oxide) deposited on inner surfaces of the substratesfacing the polymer medium 14.

As shown schematically in FIG. 1A, solidifying the medium 14 in amagnetic field perpendicular to the viewing surface 16 forces the liquidcrystal molecules 30 in the microdomains 12 to align parallel to themagnetic field. The smectic planes 32 and the electrical dipoles 34 ofthe liquid crystal 12 align randomly from microdomain to microdomain. Ifthe liquid crystal and polymer are selected such that an index ofrefraction of the polymer is approximately equal to the index ofrefraction of the liquid crystal parallel to the long molecular axis,the device 10 in an as-formed state is optically homogeneous andtransmissive along a direction perpendicular to the viewing surface 16.The scattering of incident light incident along a direction oblique tothe viewing surface 16 of the device 10 (i.e., haze) can be reduced ifthe medium 14 is birefringent and the ordinary and extraordinary indicesof refraction of the polymer are approximately equal to the indices ofrefraction of the liquid crystal parallel and perpendicular to the longmolecular axis.

For liquid crystalline materials with positive dielectric anisotropy, arelatively weak DC voltage difference generated between the transparentelectrodes 22 and 24 realigns the electrical dipoles 34 perpendicular tothe viewing surface 16. As shown schematically in FIG. 1B, the molecularlong axes of the molecules 30 in the microdomains 12 are now randomlyaligned from microdomain to microdomain. The effective indices ofrefraction of the microdomains 12 along the direction perpendicular tothe viewing direction 16 are no longer approximately equal to theeffective index of refraction of the polymer medium 14. This mismatch ofthe indices of refraction gives rise to a strong Rayleigh scattering ofincident light. The optical contrast between the light transmissiveas-formed state and the light scattering state enables the device tomodulate light.

If the liquid crystal and polymer are selected such that the liquidcrystal phase in the microdomains 12 strongly anchors at the surfaces ofthe microdomains, the liquid crystal will return spontaneously to thelight transmissive as-formed state shown in FIG. 1A when the voltagedifference across the transparent electrodes 22, 24 is removed. By thismeans, a monostable "normally transmitting" (or "reverse mode") deviceis formed. Depending on the materials used, the switching time for thisdevices may be at least approximately two orders of magnitude less thanthe typical switching times of comparable devices using polymerdispersed nematic liquid crystal.

A different selection of liquid crystal and polymer such that the liquidcrystal in the microdomains 12 weakly anchor at the surfaces of themicrodomains results in a device which will remain in the scatteringstate shown in FIG. 1B even after the voltage difference between theelectrodes 22, 24 is removed. A high frequency AC field or, in the caseof liquid crystal materials with low polarization density, a relativelyhigh DC field across the electrodes 22, 24 realigns the molecules 30back to the as-light transmissive as-formed state shown in FIG. 1A. (Thestrength of a DC field effective to realign the molecules 30 back to thestate shown in FIG. 1A is determined by the relative strengths of thepermanent and induced dipole moments.) By this means, a bistable deviceis formed. As with the normally transmissive device, the switching timefor this devices may be at least approximately two orders of magnitudeless than the switching times of typical polymer dispersed nematicliquid crystal devices. Neither the normally transmissive device nor thebistable device illustrated in FIGS. 1A and 1B require polarizers tomodulate light.

For liquid crystalline materials with negative dielectric anisotropy, aDC voltage difference generated between the transparent electrodes 22and 24 realigns the electrical dipoles 34 perpendicular to the viewingsurface 16. As in the case of liquid crystals with positive dielectricanisotropy, the effective indices of refraction of the microdomains 12along the direction perpendicular to the viewing direction 16 are thenno longer approximately equal to the effective index of refraction ofthe medium 14, giving rise to scattering. Whether the device ismonostable or bistable depends on whether the liquid crystal and polymerare such that the anchoring of the liquid crystal at the surfaces of themicrodomains 12 is strong or weak.

FIGS. 2A, 2B and 2C illustrate shutters or displays 40 comprising alight modulating material in which droplets 42 of the liquid crystalform in a polymer medium 44 during phase separation due to the influenceof external fields (as discussed below). The device 40 includes, inaddition to the light modulating material, transparent substrates 48, 50having transparent electrodes 52, 54 deposited on inner surfaces of thesubstrates facing the polymer medium 44 and polarizers 56, 58 adjacentto opposite surfaces of the substrates. The polarizers 56, 58 can berotated to have their axes parallel of perpendicular to each other,depending on requirements.

As shown schematically in FIG. 2A, solidifying the medium 44 in amagnetic field approximately parallel to the viewing surface 46 and a DCor slowly varying AC voltage perpendicular to the viewing surface 46forces the molecule 60 in the microdomains 42 to align in "bookshelf"geometry such that the smectic planes 62 are perpendicular to theviewing surface 46. The electric dipoles 64 of the droplets 42 alignperpendicularly to the viewing surface 46, while the molecules 60themselves align at a tilt angle β with respect to a line perpendicularto the smectic planes 62. (The molecules 64 are tilted out of the planerepresented by the paper, as shown by the larger end of the "molecules"shown in FIG. 2A) if the liquid crystal and polymer are selected suchthat an index of refraction of the polymer is approximately equal to theeffective index of refraction of the liquid crystal along the cell axiswhen its molecules are aligned at a tilt angle β, the light modulatingmaterial in an as-formed state transmits incident light and rotates thepolarization direction of the incident light along a directionperpendicular to the viewing surface 46.

If, in addition, the liquid crystal and polymer are selected such thatthe liquid crystal phase in the microdomains 42 weakly anchors at thesurfaces of the microdomains, applying a DC field in a directionopposite to that in which the medium 44 was solidified causes themolecules 60 to switch within the smectic layers so that tho dipolemoments 64 point in the opposite direction. In this alignment, shownschematically in FIG. 2B, the light modulating material also transmitsand rotates the direction of polarization of the incident light along adirection normal to the viewing surface 46. The rotation of thepolarization direction is opposite to that of the as-formed state. Theliquid crystal may be returned to the alignment of FIG. 2A by againapplying a DC field in the direction of the field in which the medium 44solidified.

By rotating the polarization directions of the polarizers 56, 58, it ispossible to maximize light transmission through the device 40 in one ofthe alignments shown in FIGS. 2A, 2B and minimize light transmissionthrough the device in the other alignment. By this means, a"birefringence" mode device is formed. The relative orientations of thepolarizers for obtaining maximum contrast is dependent on factorsincluding the birefringence of the smectic phase, the thickness of thelight modulating material, and the tilt angle of the liquid crystal.Advantages of such birefringence mode devices include ultra-high speedbistable switching and high contrast. One disadvantage is low lightthroughput due to the use of polarizers.

Alternatively, if the liquid crystal and polymer are selected such thatthe liquid crystal has positive dielectric anisotropy and the liquidcrystal phase in the microdomains 42 strongly anchors at the surfaces ofthe microdomains, generating a strong DC voltage across the electrodes52, 54 induces a realignment of the liquid crystal molecules toward adirection nearly perpendicular to the viewing surface 46. The degree ofrealignment is dependent on the interaction energy of the permanentdipole, the magnitude of which is dependent on the first power of theelectric field strength, and of the induced dipole, the magnitude ofwhich is dependent on the second power of the electric field strength.In this realigned state, shown schematically in FIG. 2C, the effectiveindices of refraction of the microdomains 42 along the directionperpendicular to the viewing direction 46 are no longer approximatelyequal to the effective index of refraction of the medium 44. Thismismatch of the indices of refraction gives rise to a strong Rayleighscattering of incident light. When the strong DC field is removed, theliquid crystal spontaneously returns to the alignment shown in FIG. 2A.By this means a scattering-transmissive device may be formed.

While polarizers 56, 58 are shown in FIG. 2C for purposes of consistencywith FIGS. 2A and 2B, the light scattering occurs in the state shown inFIG. 2C even when unpolarized light is incident on the medium 44. Infact, the contrast and the light throughput between the lighttransmitting state of FIG. 2A and the light scattering state of FIG. 2Cwould increase if the polarizers were removed. While ascattering-transmissive device constructed using the material shown inFIGS. 2A and 2C having polarizers such as those shown at 56, 58 would beoperative, such a device would preferably have no polarizers.

Devices operating in scattering-transmissive or bistable modes similarto those illustrated in FIGS. 1A and 1B may also be formed bysolidifying the medium 14 in the presence of an DC or AC electric field.Alternatively, devices operating in the scattering-transmissive orbirefringence modes discussed in connection with FIGS. 2A, 2B and 2C maybe formed by shearing the light modulating material while keeping theliquid crystal in either a smectic A or smectic C phase by heating, orby solidifying the medium 44 in the presence of a temperature gradientparallel to the viewing surface 46. Depending on the materials, thecircumstances in which the polymer is solidified and manner of aligningthe liquid crystal, the microdomains may be non-spherical (e.g. take theform of spheroids rather than spheres.) This difference in shape isexpected to impact on the switching behavior of the material and can beexploited to improve the operation of the device.

All of the above discussion, although framed in terms of a deviceutilizing smectic C* liquid crystal, is applicable also to devices usingsmectic F*, I*, G* and H* phases. Since these phases are capable of morethan two stable orientation when aligned in bookshelf geometry, it isanticipated that devices using these materials will be capable ofmultistable switching in a preferred configuration.

The alignment may be performed when the solution of liquid crystal isfirst formed and solidified. Alternatively, the alignment may beperformed by heating a material already containing dispersedmicrodomains of ferroelectric smectic liquid crystal and thenresolidifying the medium in the presence of an induced force to promotethe alignment of the liquid crystal.

The preferred embodiment of the invention is further exemplified by thefollowing non-limiting example:

The ferroelectric material ZLI-3234, available from E. Merck ofDarmstadt, Germany was dissolved with polymethylmethacrylate ["PMMA"] inchloroform in the following proportions:

    ______________________________________                                               ZLI-3234       0.42 g                                                         PMMA           0.28 g                                                         CHCl.sub.3     6.3 g                                                   ______________________________________                                    

The solution was put in a glass tube and mixed with an agitator for tenminutes. Spacers of 5 μm diameter were added to this mixture to provideuniform cell spacing. The solution was coated on one indium-tin-oxide["ITO"] coated glass plate with a barrier layer of SiO₂. The glass platewas left overnight to allow the solvent to evaporate, leaving a thinlayer comprising droplets of liquid crystal dispersed in transparentpolymer. The second plate was put on top of the coated one and the twowere clamped together. The cell was heated to 150° C., put under apressure of 20 psi in a hot press, and then cooled at a rate of ˜1°C./min to 30° C.

The SiO₂ was removed from small areas of the glass plates outside thecells and wire leads. Were soldered to the ITO surface with indiummetal. An electrical signal comprising alternating positive and negativesquare pulses of variable period and amplitude was applied and theswitching characteristics of the cell observed under a polarizingmicroscope.

The observations were made while changing the amplitude of theelectrical pulses having amplitudes from 0 volts to 25 volts, and pulsedurations from 10 Hz to 100 Hz. Almost 80% of the microdroplets formedaccording to the procedure set forth in the last two paragraphs werefound to respond to the electrical pulses. Nearly 10-20% of the dropletswere identified to be switching bistably. The threshold voltage forvarious droplets varied from 5 volts to 25 volts, demonstrating thepossibility of grey scale in such devices. The bistability of the ON andOFF states was confirmed by removing the leads from the source of theelectrical signal and shorting the leads to remove any charge left overon the cell plates. Shorting the leads did not affect the state of thedroplets identified to be bistable as they remained in the state inwhich they were when the leads were disconnected. A large fraction ofother droplets were found to change their optical appearance but did notappear to switch when the electrical pulses were applied, because theiroptical axes were in the wrong orientation (for the experimental set-up)to exhibit bistability.

The bistable droplets appeared to switch at a frequency of at least 100Hz, It is believed that the actual switching time was much shorter (˜100μs) as specified by the manufacturer of the liquid crystal for bulksamples, but the experiment was not designed to test switching at suchhigh speeds.

Many modifications and variations of the invention will be apparent tothose skilled in the art in light of the foregoing disclosure.Therefore, it is to be understood that, within the scope of the appendedclaims, the invention can be practiced otherwise than has beenspecifically shown and described.

I claim:
 1. A method for forming a light modulating material comprisingthe steps of:a) forming a solution having a mesogenic component capableof forming a ferroelectric smectic liquid crystal phase and amedium-forming component capable of forming a light transmissive polymermedium; b) inducing a force in the solution to promote alignment ofmolecules of the mesogenic component along a direction relative to asurface of the solution; and c) solidifying the medium-forming componentwhile the force is induced to promote phase separation of microdomainsof the ferro-electric smectic phase from the polymeric medium.
 2. Amethod according to claim 1 wherein the step of inducing a force in thesolution is performed by imposing an electromagnetic field in thesolution while the medium-forming component is being solidified.
 3. Themethod of claim 2 wherein the step of imposing an electromagnetic fieldincludes imposing an electromagnetic field having a electric fieldcomponent perpendicular to the surface of the solution.
 4. The method ofclaim 2 wherein the step of imposing an electromagnetic field includesimposing an electromagnetic field having a magnetic field componentparallel to the surface of the solution.
 5. The method of claim 4wherein the step of imposing an electromagnetic field includes imposingan electromagnetic field having a magnetic field component parallel tothe surface of the solution and a DC electric field componentperpendicular to the surface; andwherein the method includes theadditional steps of positioning optical polarizers adjacent the materialand adjusting the polarization directions of the polarizers to maximizeor minimize light transmission.
 6. A method according to claim 1 whereinthe step of inducing a force in the solution is performed by imposing athermal gradient along a direction parallel to the surface of thesolution while the medium-forming component is being solidified.
 7. Amethod according to claim 1 wherein said step of inducing said force isperformed by shearing said material to promote elongation of droplets ofthe ferroelectric smectic phase.
 8. A method according to claim 7wherein the mesogenic material is selected to have positive dielectricanisotropy.
 9. A method according to claim 7 wherein the step ofshearing the material is performed while heating the material to atemperature in which the liquid crystal is in the smectic A or smectic Cphase.